Nearly 90% of the human genome is composed of non-coding regions that are not translated into proteins. Previously referred to as "junk" DNA it has recently become apparent that some of this DNA is used to make small app. 22-nucleotide single stranded RNAs known as micro-RNAs (miRNAs). Within the last several years researchers have discovered that these miRNAs are not only important for a variety of processes, but essential. Indeed, removing miRNAs early in development by knocking out the key RNAseIII enzyme required for their synthesis, Dicer, is lethal at the very early stages of gastrulation. Importantly, these miRNAs are continuously expressed throughout the life of mammals and it is clear that they continue to play important regulatory roles in cellular function. The greatest variety of miRNAs are expressed in the nervous system, including the cortex, making it paramount to understand what role the miRNAs play in normal cortical function and mental and behavioral disorders. To date only a few miRNAs has been investigated in hippocampal neurons in culture and these miRNA was found to be required for proper neurite outgrowth and synapse morphology. Importantly computational analyses of likely targets indicate numerous signaling pathways that impact neuronal survival and function are regulated by miRNAs. We propose to use the power of mouse genetics to specifically remove all miRNAs from pyramidal cortical neurons after they have differentiated using a conditional knock out of Dicer and another key gene for the miRNA pathway DGCR8 and then ask what role miRNAs play in dendrite morphology, synaptic spines, and function. Finally, we will identify the complement of miRNAs that are expressed in purified pyramidal neurons that may underlie the observed phenotypes. These data will provide the first inroad to understanding the role of small RNA biology in normal brain function and disease. These initial studies will allow us to develop and perfect the tools to extend these studies to other, layer specific, pyramidal neurons in the cortex and to specific miRNAs that we have identified as candidates for impacting synaptic transmission or affecting schizophrenia-linked genes. PUBLIC HEALTH RELEVANCE. The small RNA pathway is a poorly understood, but essential translational repression pathway heavily expressed in mammalian neurons. This pathway is likely to be important for normal neuronal function and is almost certainly affected in a variety of behavioral and mental disorders such as autism and schizophrenia. These studies will be among the first to provide a detailed cellular and molecular description of the small RNA pathways in normal neuronal development and function in vivo.